Often, off-road trucks are subjected, during their routine use, to weight loads which differ greatly because of different material density and/or the ability of some material to more tightly pack when loaded into the truck body. In the working environment of a coal mining site, a truck may be called upon to haul either coal or overburden. Overburden is the earth material which must be removed in strip mining to expose the coal for removal. Typically, the overburden material has a greater density than the coal it covers. As a result of this difference in density, truck bodies which are always filled to their full volume capacity may carry weight loads which exceed the weight capacity of the truck.
By reducing the volume capacity of the truck body, the weight overloading problem can be eliminated. But, by reducing the volume capacity of the truck body, the truck hauls less than its full weight capacity of the lighter density coal. Therefore, the mine operator faces the unpleasant choice of either operating his trucks inefficiently (i.e., smaller volume bodies which hold less coal by weight than the truck is capable of holding) or, alternatively, risk overloading the truck (i.e., larger volume bodies which hold the full weight capacity of coal but which can be weight overloaded if fully loaded with heavier overburden).
One solution to the foregoing problem has been to employ a fleet of trucks comprising two types of truck bodies. One group of trucks is equipped with with smaller volume bodies for use in connection with hauling overburden. A second group of trucks is equipped with larger volume bodies for use in connection with hauling coal. Unfortunately, the dedication of a truck to a particular type of hauling task (i.e., hauling overburden or coal) reduces the versatility of the operator's fleet. During particular times of a mining cycle there may be a need for more trucks to haul overburden and at other times a need for more trucks to haul coal. When these times occur, the mine operator is in a situation where he must use a truck intended to haul one type of load (e.g., overburden) to haul another type of load (e.g., coal). Obviously, dedicating the truck fleet to groups intended for different types of hauling reduces the flexibility of the fleet and introduces the possibility of inefficient operation of the truck fleet.
One prior art solution to the problem of reduced flexibility of a truck fleet using truck bodies of different volume capacities has been to add a tailgate to the smaller capacity bodies in order to provide the bodies with means to increase their capacity if desired. Specifically, off-road, heavy-duty trucks are commonly equipped with bodies having beds which are angled with respect to a horizontal plane. The truck beds slope from a high point at the rear of the truck bodies to a low point at the front of the bodies. By providing bodies having the foregoing beds, loads can be carried by the bodies without necessitating separate tailgates for the bodies. By adding tailgates to the bodies, the load carrying capabilities of the bodies can be substantially increased. Therefore, many mine operators have equipped their truck fleet with smaller volume truck bodies which include tailgates for increasing the volume capacities of the bodies. When the tailgates are not used to retain loads, the trucks are operating with a smaller volume body suitable for safely hauling overburden. By using the tailgates to retain loads, the trucks are operating with a larger volume body suitable for the efficient hauling of coal.
Unfortunately, the addition of a tailgate to a truck body makes it difficult to control the exact loading of the truck when overburden is being hauled. Without the tailgate, the volumetric loading limit could be easily determined by visually monitoring the level of overburden by way of the rear opening of the truck body. With the addition of a tailgate, the loader operator can no longer make a visual estimation. Instead, he must now guess at the appropriate load.
In addition to the foregoing disadvantages of a tailgate, the tailgate also hinders the loading of a truck since a loading shovel must lift its bucket above the lowered tailgate and down into the bed of the body. For loading coal, this up and then down loading movement of the shovel is justified because of the additional load capacity provided by the tailgate. But, when the body is carrying overburden, the tailgate is not providing any additional capacity. In fact, since the loading shovel must raise the heavy buckets of overburden an additional height equal to that of the tailgate (e.g., approximate seven feet), the tailgate is actually a hindrance to the hauling of overburden. Moreover, because the tailgate requires overburden be raised relatively high above the body bed in order to clear the tailgate and load the truck, a risk is created that the overburden will be dropped into the truck body from a height which may, with continual usage, damage the truck frame and/or body.